1. Field of the Invention
The present invention relates to a control apparatus for a stepping motor used for driving a chemical pump, more particularly, a control apparatus which is capable of controlling a chemical pump so that a driving torque of the chemical pump changes in one cycle.
2. Description of the Related Art
A chemical pump for injecting a chemical into a human body is required to drive a chemical injecting pump so that the chemical is allowed to be injected into a human body at a constant flow.
The inventors of the present invention know that a chemical pump normally utilizes a stepping motor and there has been proposed a control apparatus for controlling a stepping motor used for driving a chemical pump.
Referring to FIG. 1, a known control apparatus for controlling the stepping motor used as the chemical pump will be described in the following part.
The known control apparatus for controlling the stepping motor used for driving a chemical pump is so arranged that the stepping motor 21 is controlled in a manner that the chemical pump (not shown) may be driven in a step-wise manner.
A rotation of the stepping motor 21 is sensed by a photo interrupter 22 which is connected with a central processing unit (CPU) 23. The CPU 23 determines whether or not the stepping motor 21 loses its synchronism in accordance with a signal sensed by the photo interrupter 22. In addition, the CPU 23 controls the driving current for the stepping motor 21.
FIG. 2 is a plot showing a characteristic of the stepping motor 21. The plot indicates that the torque generated by the stepping motor 21 is proportional to the driving current (motor current).
That is, as the driving current becomes larger, the power consumption becomes larger. Hence, considering the torque and the power consumption, it is necessary to utilize the CPU 23 in a manner that the stepping motor 21 is operated according to the steps of the processes shown in FIG. 3.
As shown in FIG. 3, the stepping motor 21 is started with a relatively high driving current (step T1). Then, the number in a counter n is reset to "0" (step T2). The number in the counter n indicates how often the driving current is changed. At a next step, it is checked whether or not the stepping motor 21 loses its synchronism depending on the signal sent from the photo interrupter 21 (step T3).
In a case that a result of the above step T3 is yes, then, the driving current is a bit increased (step T4) and it is determined whether or not the increased current is larger than a predetermined value (step T5). In a case that a result of the above step T5 is no, then the process returns to the step On the contrary, in a case that a result of the above step T5 is yes, then the error processing is executed.
At the step T3, in a case that the stepping motor 21 does not lose its synchronism, then the number of the counter n is incremented (step T6) and it is determined whether or not the value of the number of the counter n is equal to or larger than a predetermined value N (step T7). Depending on the determined result at the step T7, in a case that the value of the number of the counter n is equal to or larger than a predetermined value N, then the operation of the stepping motor 21 is continued. On the other hand, in a case that the value of the number of the counter n is less than the predetermined value N, then the motor current is bit decreased (step T8), and the process is returned to the above-mentioned step T3 for further operation.
Referring to FIG. 4, the known control apparatus for the stepping motor actuates the stepping motor 21 with a relatively large driving current. Then, the driving current is gradually reduced N times. It results in keeping the driving current of the stepping motor 21 substantially constant. In a case that the stepping motor 21 loses its synchronism, the driving current is a bit increased so as to amend the loss of synchronism of the stepping motor 21.
The known control apparatus for the stepping motor used for driving the chemical pump, however, has a shortcoming in that the loss of the driving current is large, because the stepping motor 21 is driven with such a substantially constant driving current so as to keep the driving torque at maximum in order to prevent the loss of synchronism of the stepping motor.